You should be doing “super curricular activities”, academic things beyond the a level syllabus. This could be attending the Cambridge university engineering masterclasses, attending other lectures/ talks/ events, reading books or articles, watching documentaries, listening to podcasts, etc. Also I would think that having some engineering work experience is important as well, especially as many other universities have a year in industry. You should start looking at different companies or contacting them as well as see what they can offer.